A lot of things in computer science that are conceptually functions actually aren’t, in this context when one compiles e.g. a C program there is a bunch of stat

A lot of things in computer science that are conceptually functions actually aren’t, in this context when one compiles e.g. a C program there is a bunch of state, some pretty counter-intuitive, that the resulting object files or executable artifacts to change even though the source code did not. This has serious implication for relying on computer systems. When a more systems-inclined person in computer science uses the word idempotent they very often mean something like “repeatable” or “deterministic”. AFIAK this usage first gained traction in the theory of distributed systems, in which it (roughly) means that one application of an operation might change the state of the system, but subsequent applications will not change it further. Set union is sort of the canonical example.